Optical polarization of nuclear spins in gallium arsenide

There is considerable interest in the control of atomic spin states. Strong, macroscopic alignment of nuclear spins within a material would yield a dramatic improvement in the sensitivity of Nuclear Magnetic Resonance (NMR) spectroscopy, and facilitate the storage and manipulation of quantum information in computing or spintronic devices. We have investigated the physical origin of optical control of nuclear spin alignment in bulk GaAs.; The process of optical nuclear polarization involves excitation of GaAs at low temperature (<50 K) with polarized near-band-gap light. Once this light excites spin-polarized electrons, it is an empirical fact that these electrons transfer polarization to nuclei. The specific electronic states responsible for the polarization transfer are the subject of this investigation.; We have measured the dependence of NMR signal enhancements on the photon energy of exciting light between 1.48 and 1.60 eV (the band gap of GaAs is 1.52 eV). The dependences of optically pumped NMR intensity on photon polarization and sample temperature vary significantly with excitation photon energy. The data suggest that some NMR enhancements (excitation well below the band gap) are due to electrons confined to defect states and other enhancements (excitation above the band gap gap) are due to electrons occupying mobile or delocalized states.; The time dependence of NMR enhancements have helped elucidate the role of electronic localization. The importance of delocalized electrons in the polarization transfer process is suggested by the linear growth of signal intensity with time for irradiation between 0.5 s and ∼1000 s and the lack of light-induced shift (or broadening) of the NMR spectrum. Independent measurement of the time dependence of average nuclear spin polarization (through quadrupolar NMR measurements of strained crystals) has shown that nuclear polarizations are spatially inhomogeneous. The spatial distributions of electron density required to fit the data, however, are larger than would be expected if defect-bound electrons were the sole sources of nuclear magnetization. We therefore argue that optical nuclear polarization is largely a bulk phenomenon.; Electrons confined to shallow defect states can be promoted into the conduction band through application of electric fields across the sample. (Abstract shortened by UMI.)...